Electronic circuits are widely used in communication devices. In particular, mobile devices that rely upon modulated electromagnetic radiation to communicate a signal require filters to remove noise and interference from received electronic signals and, in particular, to remove frequencies outside a desired range. Such filtering can be done in a variety of ways, for example, with electromechanical filters, with resonant electronic circuits such as tuned resonant tank circuits including various combinations of resistors, capacitors, and inductors, and with digital filters using digital signal processors to filter digitized electronic signals.
Electromechanical filters have been used extensively in radio communications for many decades. One group of such filters depend upon electromechanical piezoelectric materials that mechanically (acoustically) resonate at a desired frequency and that either respond mechanically to electrical stimulation or produce an electrical signal in response to mechanical stimulation. Electromechanical filters using piezoelectric materials are variously known as acoustic filters, acoustic wave filters, acoustic resonators, crystal filters, or crystal oscillators. A variety of useful piezoelectric materials are known. For example, quartz has been used for more than 80 years because of its low coefficient of thermal expansion and high quality factors.
In recent years, different acoustically resonant modes have been exploited in piezoelectric materials, including surface acoustic wave (SAW) filters and bulk acoustic wave (BAW) filters. U.S. Pat. Nos. 5,313,177, 7,307,369 describe surface acoustic wave devices. U.S. Pat. No. 5,872,493 teaches a bulk acoustic wave (BAW) filter having a protective acoustic mirror. Acoustic mirrors are also known as reflector layers or acoustic reflectors. Single crystal acoustic resonators (SCARs) are also known, for example, as disclosed in U.S. patent application Ser. No. 14/796,939.
In many applications, for example, mobile communication devices found in cellular telephones, size and weight are important device attributes and therefore electronic circuits are preferably small, light, low cost, and highly integrated. Electromechanical filters are used as components in electronic circuits and there is, therefore, a need for improved integration of such filters in electronic circuits for telecommunication devices.